Existing fiber optic transmission media have employed single mode fiber coupled to free space. Standard single mode fiber produces a Gaussian field radiation profile and a Gaussian far-field beam.
Applications of transmitted optical beams include infrared countermeasures (IRCM), free space optical communications (FSO Com) and laser cutting and drilling for manufacturing.
In the cases of IRCM and FSO Com, a transmitted laser beam is directed to a distant position. The imperfect accuracy of directing the beam requires distributing the transmitted energy across a large enough solid angle that the extent of the receiver will be illuminated. The link budgets which describe the systems require a minimum power level at the receiver. If that power level is exceeded, the additional power is unnecessary and wasted. At present, beam profiles are Gaussian. This is the result of using a single mode fiber (SMF) in the transmit path—the mode within SMF is Gaussian. The width of the far-field Gaussian beams are such that at the edge of the required solid angle, the power must exceed a required amount. The result is excess power being transmitted on bore sight and unused power being transmitted outside the region of interest.
Known applications require that light directed into the optical fiber not fill it; the numerical aperture is smaller than the intrinsic numerical aperture of the optical fiber. They also rely on pure silica cores and require two optical fibers. Fiber lengths are also between 0.2 and 10 meters. Some applications employ graded and step index fiber optic cores or include mode mixers.
In laser beam welding and drilling, a spot is focused on the surface of the work piece. Melting begins and the light energy is converted into thermal energy. For welding, the surface melts and the beam energy is held below the vaporization point of the material. Vaporization is employed for drilling and cutting. Energy distribution across the beam is typically determined by the design of the resonant cavity. This includes mirror curvature, shape and arrangement. It results in photon oscillation within the cavity, creating Transverse Energy Modes (TEMs). Gaussian mode is designated as TEM00. Focusing lenses are important in delivering energy to the work material. References such as Joining Technologies' technical article “Laser Beam Welding”, found at http://www.joiningtech.com/news/laserbeamwelding—24/, specify that beam profiles should have a Gaussian peak energy at the beam center.
Each application exhibits inefficient energy delivery to the target. Disadvantages are that the Gaussian profile wastes power for infrared countermeasures (IRCM) and free space optical (FSO) communications and limits accuracy for laser drilling and cutting.
What is needed, therefore, are techniques for more efficient transmission systems to more closely match application needs.